1. Field of the Invention
This invention relates generally to equipment and techniques for applying rotary motion and torque to objects. In a specific application, the invention relates to the use of hydraulically powered wrenches for screwing two threaded oilfield tubulars to each other and applying and then holding a precise torque on the screwed together connection.
2. Brief Description of the Prior Art
Oilfield tubulars such as casing and tubing are typically thirty to forty feet in length and have threaded connections at their ends. The pipe sections, or "joints", are usually screwed together with hydraulically powered wrenches, referred to as "power tongs", to form a long pipe string. The tongs are used on a drilling or completion rig to add the joints to a pipe string which is lowered into the well. Pipe strings are used to case the well, bring well fluids to the surface, control the well and in some cases to drill or workover the well. Power tongs are also used in pipe threading facilities and pipe yards to "buckon" or screw on couplings to the threaded ends of pipe joints.
This invention is concerned in part with the screwing together or "makeup" of threaded connections which employ internal shoulders or metal-to-metal sealing surfaces which limit relative rotation between mating connections after the shoulders or surfaces engage. The two threaded pieces may be screwed together with relatively low torque until the shoulders or seals in the connection engage. Continued effort to turn the pipe after shouldering causes the torque applied to the connection to increase very rapidly with only a small amount of additional rotation.
One benefit of these connections is that they allow the string to be rotated in the well without continued screwing together of the pipe segments. Application of high torque to the connections can also preload the seals so that they remain tightly engaged when the string is hung in tension. Hoop and compressive stresses in the connection are also significantly less than those present in conventional, tapered, interference connections.
Torque must be precisely applied to these special connections for proper makeups. This can be difficult to achieve with conventional tongs because the torque climbs so rapidly after shoulders or seals in the connections engage. The tongs may be controlled to prevent over torquing with a system which automatically bypasses or "dumps" the hydraulic power fluid around the tong motor when a desired minimum torque level is sensed. However, the response time in these dump systems is relatively slow compared to the rise time of the torque applied to the connection. The result can be poor control over the final torque applied to the connection.
Another problem with most dump systems is that a very quick, sharp spike or pulse of torque which reaches the minimum torque level even instantaneously may cause the system to dump even though the connection may not have had sufficient time to respond to the torque.
Controlling conventional tongs by adding means to closely control the fluid pressure or the fluid flow through the tong motor produces only limited improvement. Even if a particular combination of hydraulic power source and tongs may be adjusted so that fairly consistant results are obtained, the control is lost if the temperature or viscosity of the hydraulic fluid change or if a tong or power source is replaced. As a further problem, the output power from the tong motor is nonlinear and if the tong motor is at the peak of its power cycle when shouldering occurs, the torque output of the tongs is greater than at some intermediate point of its power cycle. The power cycle position of the motor at the shouldering point cannot practically be controlled from one makeup to the next. The problem is compounded when it is desired to reach and then hold a selected torque value. The control mechanisms which are designed to dump torque at a desired value are not designed to hold or keep the torque constant at the desired value.
Prior art attempts to correct these problems include systems which use specially designed control mechanisms built into the tongs to enable a slow, controlled application of torque. With such equipment, the torque values may be held above a selected minimum torque value for a sufficient time to ensure full application of torque to the connection. When this approach is taken, control is limited by the particular drive and power mechanism used in the tongs and appropriately modified tongs are required for every different range of pipe sizes. Moreover, the cost of modifying existing conventional tongs and the number of modified tongs required to cover the entire range of pipe sizes renders the prior art approach relatively expensive. Additionally, since the control mechanism is an integral part of the tongs, it is necessary to transport the tongs to the job site rather than use tongs which might already be at the site. This can cause delay and additional transportation expense.
Another problem associated with prior art power tongs is their inability to accurately apply very low torques required for some connections. For example, some fiberglass connections require makeup torques as low as 100 foot-pounds or less. Most small tubing tongs are designed to apply torques in the range of 1,000 to 7,000 foot-pounds. Because of the size and weight of the tongs and because of characteristics of their power supplies and motors, accuracy in applying very low torque values is difficult if not impossible. The problem is present at these low torques for both shouldering connections and conventional interference connections which have no internal shoulders.